Optical disks have become widely used in part due to their relatively high storage capacity and long storage life. Whereas a 31/2 inch floppy disk can store 1.44 Mb (megabytes) of data, a 12 centimeter compact (optical) disk can store upwards of 650 Mb. Optical disks have therefore become increasingly popular in recent years, with a number audio and computer data storage products becoming commercially available in a compact-disk (CD) optical disk format. More recent developments in the application of optical disk technology, such as the MultiMedia CD (MMCD) developed by Sony Corp., the Super Density (SD) system developed by Toshiba Corporation, and the new Digital Video Disk (DVD) standard provide ever increasing storage capacities.
A read-only compact disk (CD-ROM) generally includes a 1.2 mm transparent plastic substrate having data encoded in pits that are impressed into the substrate along spiral or concentric data tracks on a 1.6 micron pitch. The pitted substrate is coated with a reflective layer.
To retrieve information from an optical disk, the disk is rotated and light is directed through the transparent substrate onto the reflective layer. The inhomogeneity created in the reflective surface by the pits causes light reflected from the pitted reflective surface to be of lower intensity, while the non-pitted reflective surface between the pits reflects almost all of the incident light. It is this modulation in light intensity that is used for information storage and retrieval.
Light reflected from the optical disk is directed to image detectors comprised of photosensitive elements that develop electronic signals corresponding to the intensity of the reflected light. These electronic signals are decoded by processing circuitry that recovers the digital information stored in the data tracks of the optical disk. Further details regarding the construction and use of optical disks in can be found in Compact Disc Technology, Nakajima, H. and Ogawa, H., translated by Aschmann, C., published by Ohmsha, Ltd., Japan (1992), and The Compact Disc Handbook, Pohlmann, K., 2d. ed., A-R Editions, 1992.
Because data is encoded sequentially along the data tracks of an optical disk, optical disk reading apparatus must be able to follow a particular track in order to be able to read the data encoded thereon. One of the problems encountered in reading data from an optical disk is that the data tracks typically exhibit eccentricity, which may cause the distance between a data track and the disk center to vary by as much as 70 microns. To read information from an optical disk, the apparatus must be able to accommodate this eccentricity during the retrieval of information.
In previously known optical disk readers, the image detector is mounted on an optical pickup which reads from one data track at a time. The optical pickup typically includes lenses for focusing the light from the light source to particular portions of the disk surface, and for reflecting light from the reflective disk surface to the image detector, as described, for example, in the aforementioned Compact Disc Technology text at Chapters 6 and 7, which are incorporated herein by reference.
In such previously known systems, tracking is generally accomplished using the well-known "twin spot" method, as described, for example, at pp. 133-136 of the above-incorporated text. In this method, secondary beams from the light source are projected onto the optical disk ahead of and behind the main illumination beam, and slightly off-axis from the main illumination beam. Thus, tracking detectors associated with the secondary beams "see" only the non-pitted reflective surface of the optical disk when the main illumination beam is centered over the data track. An error signal is then developed as the difference between the signals generated by the tracking detectors. That error signal is provided to a servo-motor controlling movement of the pickup.
The increased availability of CD-ROM, and the development of MMCD, SD and DVD products, coupled with the availability of increasingly faster microprocessors, has created the need for ever faster optical disk drives. As a result, disk drives capable of operating at multiples of a standard single speed drive are becoming available, for example, 2.times., 4.times. and even 6.times. drives. For a 6.times. (six times single speed) disk drive, the disk is rotated at speeds up to 2400 rpm when reading the innermost data track.
The ability to achieve even greater speeds may soon be limited by the ability of such technology to continue to provide low-cost, easily manufacturable systems, since the use of greater disk rotational speeds will require more sophisticated, higher tolerance, and thus more expensive, designs than employed in previously known arrangements.
An alternative to simply increasing the disk rotational speed is to read multiple data tracks simultaneously, as described in commonly assigned U.S. Pat. No. 5,426,623, the entirety of which is incorporated herein by reference. In accordance with the methods and apparatus provided therein, for example, ten adjacent data tracks may be read simultaneously. Thus, even if the disk is rotated at only twice standard speed (i.e., a 2.times. drive is used), the capability to read multiple tracks provides the equivalent of a 20.times. drive--more than a factor of three faster than previously known disk drives.
Implementation of simultaneous multiple track reading capability for optical disks presents new problems, however, relating to tasks such as focussing and tracking. For example, with development of MMCD and DVD formats, ever more data is being compressed onto a single disk. Thus, for example, the track pitch for the MMCD system is only about one-half that of the CD-format, and includes shorter pit lengths and faster rotation speed that increase the linear velocity from about 1.4 m/s to 4 m/s.
In "Parallel Optical Memories", BYTE magazine, September 1992, at p. 179, by Demetri Psaltis, a system is described which enables parallel access to optical disk tracks using a CCD detector to receive images formed by a wide-area illumination beam. As also described in "Optical memory disks in optical information processing" Applied Optics, Vol. 29, No. 14, May 10, 1990, by Demetri Psaltis, and in U.S. Pat. No. 5,111,445, this system encodes information in different formats in radial and tangential directions on the optical disk, so that it may be decoded without the aid of focussing optics.
In view of the foregoing, it would be desirable to provide improved electronic tracking methods and apparatus, for use with apparatus capable of simultaneously reading multiple data tracks using an image detector, that generate an electronic tracking signal based on the output of the image detector.
It further would be desirable to provide improved electronic tracking methods and apparatus, for use with apparatus capable of simultaneously reading multiple data tracks, that reduces cross-talk in the signals representative of data contained in adjacent data tracks of the optical disk.
It would be yet further desirable to provide improved electronic tracking methods and apparatus, for use with apparatus capable of simultaneously reading multiple data tracks, that permits reduction in the data readout error rate.